1) Create a free account at https://imgur.com/ where you can drag and drop your images (any size) from your computer (maybe not easy on a smartphone though). You should store the images on Imgur (not delete them afterwards) so they will always be accessible on the forum, otherwise the image will stop displaying on the forum page. This costs you nothing so it's not a hassle.
2) Click on the uploaded pic on your Imgur page and copy the 'direct link' to your clipboard:

3) Select the 'insert image' button above the window you are typing in:

4) Paste the direct link between the markers [ img] and [ /img] in your post (note there is no space after the '[' in the actual link):

5) Select 'Preview' at the bottom of your post to check that you did it all correctly and the pic displays, then submit the post to the forum.

